The present invention relates to optical combining and depolarizing technology, and in particular, relates to a hybrid component and method capable of simultaneously combining and depolarizing pumping lights from two pump laser diodes with different wavelengths and optical amplifiers utilizing the same.
Recently, polarization beam combiner and depolarizer have attracted a lot of attention because of their promising applications in Raman amplifiers. For a Raman amplifier, the amplification is not transition among energy levels, but stimulated Raman scattering. It is common knowledge that stimulated Raman scattering is highly dependent on the polarization relationship between pumping light and signals. In Raman amplification, only the signals matched with the polarization of pump light can be amplified. In other words, when the polarization orientations of the signal lights and the pumping light are vertical to each other, the signal lights can not be amplified at all; while the polarization orientations of the two are parallel, the signal lights can be amplified to a maximum level; and the amplification for other polarization relationships is between those for the above two cases. This unique amplification mechanism leads to polarization dependent gain (PDG) for the signals lights.
A semiconductor pump laser of Fabry-Perot type with wavelength stabilized by a fiber Bragg grating (FBG) has a wide spectrum width. Therefore, stimulated Brillouin scattering can be depressed effectively when the pumping laser yields several hundreds of miliwatts. Usually, the pumping light is linear-polarized output. When the gain medium is not polarization-maintaining fiber, the Raman gain will vary due to the variation of the relative state of polarization of the signal and the pumping light. In practical applications of distributed Raman amplifiers, the gain medium is the transmission fiber, for example, STM-28, which is nearly an ideally cylindrical wave guide, of which the depolarizing capability is too weak to meet the market demand. Especially for a co-pumped scheme, the PDG may be greater than 0.5 dB when the Raman gain is around 10 dB.
There are two conventional arts to achieve a depolarized pumping source as following.
To obtain a stable gain for a signal of all polarizations, people usually use two linear-polarized pumping lights combined by a polarization beam combiner (PBC) to reduce PDG, of which the polarization orientations are orthogonal. However, if the two pumping lights have significantly different spectra, the PBC can not depolarize them effectively any more. Moreover, this depolarizing approach doubles the number of pumping laser diodes for each pumping wavelength, and increase cost significantly.
A depolarized pumping light can also realized by combining two pumping laser diodes of different wavelengths using a polarization beam combiner with polarization-maintaining fiber output, and then depolarizing the combined light by a depolarizer with a polarization-maintaining fiber input. This scheme adopts a discrete polarization beam combiner (PBC) and a discrete depolarizer, resulting in greater insertion loss (1.2 dB or more), bulk size and more cost.
Some prior art documents such as U.S. Pat. Nos. 5,812,583(1998.9.22) and 5,999,544(1999.12.7) disclose that two prisms are used for depolarizing, but not integrated with another part for polarization beam combining.
Therefore it is necessary to find out an effective method to reduce PDG with low cost.
The present invention aims to provide a hybrid component and method capable of combining and depolarizing two different wavelength pumping beams with compact size, and low insertion loss, as well the optical amplifier thereof.
The present invention provides a hybrid component for combining and depolarizing pumping lights comprising: an input means for inputting two linear polarized pumping lights; a polarization beam combiner for combining said two linear polarized pumping lights to form a combined beam; a depolarizer for depolarizing the combined beam; and an output means for collecting and outputting a single combined and depolarized pumping beam. Said input means includes a dual polarization-maintaining core collimator, said polarization beam combiner includes a Wellstone prism consisting of two wedge prisms with orthogonal optical axis, and said outputting means includes a single core collimator.
The present invention further provides an optical amplifier including the hybrid component for combining and depolarizing pumping lights as defined above.
The present invention further provides a method for combining and depolarizing pumping lights in an optical amplifier comprising the steps of: inputting two polarized pumping lights; combining said two polarized pumping lights to form a combined beam; depolarizing the combined beam; collecting and outputting a single combined and depolarized pumping beam. Said combining step and depolarizing step combine and depolarize the two pumping lights simultaneously.
According to the invention, a Polarization Beam Combiner (PBC) and a depolarizer are integrated into one single hybrid component, in which two pumping lights of different wavelengths and orthogonal polarization orientations are combined and depolarized simultaneously.
With the hybrid component of the present invention, the number of the pumping laser diodes can be reduced by half compared to the first conventional art. And the size of the component and insertion loss can also be decreased greatly compared to utilize discrete components.